Performance of delay and add direct sequence spread spectrum modulation scheme with fast frequency hopping in frequency selective Rayleigh channels

The coherent direct-sequence spread spectrum (DSSS) modulation is a very attractive modulation technique for military communication systems, mainly due to its resistance to interference and low probability of detection (LPD). However, the synchronisation to the spreading code phase in the receiver takes time, which makes this technique less interesting for sending very short messages or to combine this technique with frequency hopping. Recently, a new non-coherent DSSS modulation scheme, called the delay and add direct sequence (DADS) modulation scheme, based on the transmit reference (TR) principle has been introduced. DADS has demonstrated to be particularly suited for short burst transmissions and can be combined with a fast frequency hopping scheme. In this paper, the performance of the DADS modulation scheme in combination with fast frequency hopping in frequency selective Rayleigh channels is studied. Theoretical analysis and simulation results show that the BER performance of DADS is improved drastically in frequency selective Rayleigh channels although keeping the same non-coherent receiver. Moreover, the BER performance of DADS is also enhanced with fast frequency hopping due to an increase in the diversity order of the frequency selective Rayleigh channels and provides LPD/LPI to the modulation scheme.